2) non-void specialization, used to communicate objects between threads

3) void specialization, used to communicate stateless events

The class template std::promise provides a facility to store a value or an exception that is later acquired asynchronously via a std::future object created by the std::promise object.

Each promise is associated with a shared state, which contains some state information and a result which may be not yet evaluated, evaluated to a value (possibly void) or evaluated to an exception. A promise may do three things with the shared state:

make ready: the promise stores the result or the exception in the shared state. marks the state ready and unblocks any thread waiting on a future associated with the shared state.

release: the promise gives up its reference to the shared state. If this was the last such reference, the shared state is destroyed. Unless this was a shared state created by std::async which is not yet ready, this operation does not block.

The promise is the "push" end of the promise-future communication channel: the operation that stores a value in the shared state synchronizes-with (as defined in std::memory_order) the successful return from any function that is waiting on the shared state (such as std::future::get). Concurrent access to the same shared state may conflict otherwise: for example multiple callers of std::shared_future::get must either all be read-only or provide external synchronization.